1. Field of the Invention
The present invention relates to a technique for controlling emission brightness of a light source to correct a deviation in chromaticity.
2. Description of the Related Art
In a passive type image display device which displays an image on a panel by use of light emitted from a light source to the panel, currently, a white cold cathode fluorescent lamp is mainly used as the light source.
On the other hand, a light emitting diode (hereinafter abbreviated as “LED”) has the following advantages. Specifically, the LED can emit light of high brightness. The color reproduction range of the LED can be broadened, and further the LED has a long operation life. Moreover, with an improvement in conversion efficiency, which has conventionally been low, as well as a reduction in costs, the LED has been used as a light source of the above-mentioned passive type image display device.
FIGS. 29A and 29B show a backlight device (direct type) of a liquid crystal display device, which is a passive type image display device having LEDs used as light sources. The backlight device includes LEDs 291, 292, and 293 which emit light of three different colors as red light sources (hereinafter referred to as “R light sources”), green light sources (hereinafter referred to as “G light sources”), blue light sources (hereinafter referred to as “B light sources”), respectively. A desired color image is displayed by mixing light of the three different colors into white light, and then emitting the resultant light to the liquid crystal panel disposed in front of the backlight device.
In order to allow the backlight device to function as an appropriate surface light source emitting light to the liquid crystal panel without unevenness in brightness or in color, an optical member 295, such as a diffuser, is disposed between the light sources and the liquid crystal panel, for converting light from the light sources to obtain such a desired surface light source. Since the optical member 295 is in a sheet form or a plate form so that the thickness of a completed crystal display device can be made as small as possible, the optical member 295 is gradually bent or deformed by heat from the light sources and circuit components. Accordingly, for the purpose of preventing the optical member 295 from being bent or deformed to the light source side, the backlight device is provided with projections 294 therein which project to the optical member side (liquid crystal panel side) higher than the light sources.
Here, an LED has a problem that the display performance thereof is deteriorated by changes in brightness and chromaticity, caused by the environment in which the LED is used, particularly, by the temperature. In addition, the display performance also changes in accordance with the usage period of the LED. For reference, FIG. 30 shows a graph indicating a brightness-temperature characteristic of an LED. As can be seen from FIG. 30, the brightness of an LED tends to deteriorate along with a rise in the temperature, and an influence of the deterioration in brightness is most prominent in the R light source (characteristic shown by a chain double-dashed line).
Thus, when LEDs are used as light sources, chromaticity of the light sources changes in accordance with the elapse of time and also with a rise in the temperature, thereby causing a deviation in the chromaticity. To prevent this deviation, emission brightness (intensity) of the light sources is monitored at regular intervals to correct deviations in brightness and chromaticity. In this way, the color of light to be emitted to the liquid crystal panel is maintained to be a desired color of white.
For example, Patent document 1 discloses a color display device which includes a means for adjusting the color balance (particularly the white color balance). This color display device includes a so-called field sequential backlight device in which light sources of R, G and B that emit different colors of light are switched sequentially to produce a specific color of light. In this color display device, the color balance adjustment is implemented by monitoring the brightness level of each of the light sources in the backlight device.
As shown in FIG. 31, the color display device disclosed in Patent Document 1 includes R, G and B light sources 311, 312 and 313, while optical sensors 314, 315 and 316 are respectively disposed in the vicinities of the LEDs. The optical sensors 314, 315 and 316 detect the brightness levels of the respective LEDs at the time of light emission, and then convert the brightness levels into corresponding voltage values. Then, each of the converted voltage values is compared with a corresponding reference voltage value predetermined for each of the LEDs. The color display device controls the current flowing in each of the LEDs on the basis of whether the voltage value is higher or lower than the reference voltage value, and of the difference between the voltage value and the reference voltage value. The color display device thus maintains the color balance by adjusting the brightness level of each of the LEDs.
Patent Document 1 also discloses a case of providing a single optical sensor having a selector switch for selecting the reference value, instead of providing optical sensors for the respective LEDs. By sequentially switching the selector switch, a comparison is made between a reference value predetermined for each LED and a detected value from the optical sensor, and thus each of the LEDs is controlled.
In addition, Patent Document 1 discloses the following other arrangement examples of the position of the optical sensor, under an assumption that the position shown in FIG. 31 is the standard arrangement position. In a case where there is no space on a substrate to dispose the optical sensor, the sensor may be disposed in a space provided in the reflector frame as in FIG. 32A, or be disposed on a wall surface of the reflector frame as in FIG. 32B. In a case where arrangement of the LEDs is limited so that a sensor cannot be disposed inside the LED box, the sensor may be disposed on the diffuser in a portion that does not interfere with the projected light as in FIG. 32C. Although not a device to detect actual light, a photocoupler consisting of an LED having the same property as those used in the illumination portion and the optical sensor may be disposed outside the illumination portion as in FIG. 32D. In a case where it is difficult to even dispose the photocoupler, the sensor portion may be connected with the illumination portion by optical fiber as in FIG. 32E.
Patent Document 2 discloses a configuration including a plurality of LEDs 331 emitting light of a single color, and a photo detection element 332, all of which are covered with a transparent resin layer 333, as shown in FIGS. 33A and 33b. With this configuration, the photo detection element can detect light propagated through the transparent resin layer. Hence, emission intensity can be appropriately detected with a single photo detection element disposed for the plurality of LEDs.
Patent Documents 3 to 5 disclose so-called side edge type liquid crystal display devices in which light is guided evenly to form a surface by use of a transparent light guiding plate, and the light is guided to the liquid crystal panel to display an image. In this liquid crystal display device, since a white color is obtained by mixing a plurality of luminescent colors by use of the transparent light guiding plate, an ideal white light is maintained by detecting the mixed white color and performing feedback control by use of a deviation from the ideal white light.
Patent Document 3 describes that it is most preferable to dispose the sensor on the same surface of the light guiding plate as the surface on which light from the light source is incident, since light which has traveled the longest distance through the light guiding plate and which is evenly mixed can be detected.
Meanwhile, Patent Document 4 describes that it is preferable to dispose the sensor on a surface other than the light incident surface such that light from the light source cannot be interfered.
Patent Document 5 points out a problem that when light from a specific light source enters a sensor from a light guiding plate, brightness of incident light having a certain angle is high, and an influence of this certain angle obstructs the appropriate detection of light by the planar light source. For prevention of this problem, Patent Document 5 describes provision of a light shielding member that shields light that enters at an angle not less than a predetermined angle before the light enters the sensor.
Patent Document 6 also describes a technique for detecting light by a sensor, the light transmitted through a light guiding plate as in FIG. 34. The light emitted from an LED light source 341 is made even inside the backlight 342, and light from a region including reflection dots is detected by a sensor 343. Then, according to the detection result, an emission controller 344 is configured to control emission intensity of the light source 341.
Patent Document 1: Japanese Patent Application Laid-open Publication No. Hei 10-49074
Patent Document 2: Japanese Patent Application Laid-open Publication No. 2002-344031
Patent Document 3: Japanese Patent Application Laid-open Publication No. Hei 11-260568
Patent Document 4: Japanese Patent Application Laid-open Publication No. 2004-21147
Patent Document 5: Japanese Patent Application Laid-open Publication No. 2004-199968
Patent Document 6: Japanese Patent Application Laid-open Publication No. 2004-342454